For mobile data communications, layered communication networks may be deployed as e.g. described by the 3GPP specification (3rd Generation Partnership Project). The 3GPP specification comprises three distinct layers: an application layer, a network control layer and a connectivity layer. The application layer supports end-user applications and may be implemented in mobile stations or application servers in the network. The application layer interfaces with the network layer to enable designing and implementing different services and applications. The network control layer supports communicating services across e.g. different types of networks such as circuit-switched domain networks based on the GSM standard using e.g. an ISDN-related technology or packet-switched networks employing e.g. the GPRS (General Packet Radio Service) technology. The connectivity layer is a transport layer capable of transporting any type of service via e.g. voice, data and multimedia streams.
The 3GPP specification defines the Long Term Evolution (LTE) as an access network technology. The LTE comprises a MME (Mobility Management Entity) forming a control-entity suitable for signalling and selecting a serving gateway (SGW) which routes and forwards data packets carrying e.g. multimedia services. For delivering the multimedia services across the access network, the 3GPP specification defines the IP Multimedia Subsystem (IMS). The IMS comprises a service layer forming an application layer, a control and connectivity layer forming a network control layer, and an access layer forming a connectivity layer. The control and connectivity layer comprises call session control functions (CSCF) forming central entities for Session Initiation Protocol (SIP) signalling. According to the IMS, an application server is provided for hosting and executing services for user entities. An example of an application serveris the Multimedia Telephony Service server (MMTel) offering multimedia communications such as voice. Another example of an application server is the Voice Call Continuity Server (VCC). The IMS further specifies a Home Subscriber Server (HSS), which is a master user database that supports the IMS network entities to handle calls. The HSS contains subscription-related information or subscriber profiles or user-specific data, performs authentication and authorization of user entities, and provides information about the subscriber's location and IP information.
According to the IMS, the application server interfaces with the Serving CSCF (S-CSCF) using the SIP. For establishing a communication link towards a user entity, the S-CSCF transmits a SIP invite message towards the application server. In response thereto, the application server may query the HSS for user-specific data indicating e.g. whether the user entity is accessible over a packet-switched access network or whether the user entity is accessible over a circuit-switched access network. However, if several application servers are deployed for communications, then a number of queries towards the HSS may result.